$DD00/56576/CIA2+0: Data Port A (Serial Bus, RS232, VIC Base Mem.)
+----------+---------------------------------------------------+
| Bit 7 | Serial Bus Data Input |
| Bit 6 | Serial Bus Clock Pulse Input |
| Bit 5 | Serial Bus Data Output |
| Bit 4 | Serial Bus Clock Pulse Output |
| Bit 3 | Serial Bus ATN Signal Output |
| Bit 2 | RS232 Data Output (User Port) |
| Bit 1-0 | VIC Chip System Memory Bank Select (low active!) |
| | %00, 0: Bank 3: $C000-$FFFF, 49152-65535 |
| | %01, 1: Bank 2: $8000-$BFFF, 32768-49151 |
| | %10, 2: Bank 1: $4000-$7FFF, 16384-32767 |
| | %11, 3: Bank 0: $0000-$3FFF, 0-16383 (Standard) |
+----------+---------------------------------------------------+
Default Value: $17/23 (%00010111)
Kernal-Reference:
CMP $DD00 : $ED69 $EE5D $EE6A $EEAC
LDA $DD00 : $ED2E $ED66 $ED84 $EDBE $EDF3 $EE5A $EE67 $EE85 $EE8E $EE97
$EEA0 $EEA9 $FE7B
ORA $DD00 : $F492
STA $DD00 : $ED33 $ED8B $EDC3 $EDF8 $EE8A $EE93 $EE9C $EEA5 $F495 $FDCD
$FE82
Bit 0..1: VIC Bank
%00, 0: Bank 3: $C000-$FFFF, 49152-65535
%01, 1: Bank 2: $8000-$BFFF, 32768-49151
%10, 2: Bank 1: $4000-$7FFF, 16384-32767
%11, 3: Bank 0: $0000-$3FFF, 0-16383 (Standard)